Digital representations of three dimensional (3D) scenes (or sites such as cities) are well known. Such representations are typically generated using data collected by instruments such as LIDAR (Light Detection And Ranging), IFSAR (Interferometric Synthetic Aperture Radar), photogrammetric techniques, or surveys. The collected data represents the topology of the scene. These instruments and systems produce digital terrain elevation data suitable for use in providing terrain visualization systems. However, there is a need for determining changes in these sites or scenes as they change over time (or by use of different data collection systems). That need is driven by applications such as immediate identification of new construction, rapid 3D assessment of damage done by natural forces such as hurricanes or tsunamis, by bombardment in a battle environment, and digital nautical charts.
For the scope of this document, we define a Digital Terrain Model (DTM) as a digital dataset comprised of elevations. Each elevation possesses—or has the ability to derive due to its storage format—a horizontal coordinate reference system component (such as WGS84 latitude & longitude) and a height (such as bare earth relative to WGS84 Mean Sea Level). Various implementations of DTMs constrain the horizontal spacing to be uniform (such as in a “gridded DTM” like DTED®) while others record the data in a series of triangular faces, such as in a triangulated irregular network (TIN). Still other implementations support storage of multiple heights for a single horizontal location. By convention, a DTM with uniform horizontal spacing (e.g. a “gridded DTM”) is called a Digital Elevation Model or DEM).
Additionally, for the scope of this document, we define a “3D Site Model” (or “3D Model” for short), as a polygonal representation of a scene. This polygonal representation supports the storage of objects (such as an individual building) by recording a series of elevations in a well-defined 3D geometry format such as might be used by a computer-aided design (CAD) system. In addition to offering a more complex depiction of a scene, a 3D Model offers advantages versus a DTM in that the 3D Model may additionally associate metadata with a given object (such as the owner of a building, the telephone number of the main switchboard, latest tax assessment, etc.). The downside to 3D Models is that they are typically much more labor-intensive to produce than is a DTM for the same scene.